Method and apparatus for measuring the weight of solid particles suspended in a carrier gas

ABSTRACT

The weight of particles suspended in a carrier gas is determined regardless of particle size by electrically charging the suspended particles in the carrier gas, thereafter jointly passing the carrier gas and a stream of pure gas under conditions of at least approximately laminar flow through an electric field arranged to cause a portion of the charged particles to migrate into the pure gas, and thereafter separating the initially pure gas from the carrier gas and measuring the amount of particles suspended in the initially pure gas by conventional means. It is shown that the measured values obtained are independent of the particle size and provide an indication of the weight of particles suspended in the carrier gas.

1D '1; 1 ilnited States atent 1191 1111 9 9 Sigrist Jan. 22, 197d [54]METHOD A APPARATUS D 3,ll4.877 12/1963 Dunham 73/28 X Cl 11] X 1 Filed:

PARTICLES SUSPENDED IN A CARRIER GAS Inventor: Willy Sigrist,Ennetbuergen,

Switzerland Mar. 27, 1972 Appl. No.: 238,197

Foreign Application Priority Data Mar. 29, 1971 Switzerland 4552/71References Cited UNITED STATES PATENTS 10/1959 Orretal.. 356/103 4/1960Grindell 73/28 VOL 77765 SDI/E65 DUST A7475? Primary Examiner-Ronald L.Wibert Assistant Examiner-F. L. Evans [5 7] ABSTRACT The weight ofparticles suspended in a carrier gas is determined regardless ofparticle size by electrically charging the suspended particles in thecarrier gas, thereafter jointly passing the carrier gas and a stream ofpure gas under conditions of at least approximately laminar flow throughan electric field arranged to cause a portion of the charged particlesto migrate into the pure gas, and thereafter separating the initiallypure gas from the carrier gas and measuring the amount of particlessuspended in the initially pure gas by conventional means. It is shownthat the measured values obtained are independent of the particle sizeand provide an indication of the weight of particles suspended in thecarrier gas.

6 Claims, 3 Drawing Figures 461 'SouecE I'KHZ all/STAN? PATENTEU 3,787,12.3

sum 1 BF 2 JTIIIIIIIIIITIIIIII/lll KL\\ IIIIIIIIIIIIIIII FOR G PARTICLESSUSPEND W W n' GAS This invention relates to the determination of theweight of suspended solid particles in a carrier gas, and particularlyto an improved method and apparatus for measuring the weight of solidparticles suspended in a carrier gas by means of optical apparatus.

It is known to determine the concentration of solid particles suspendedin air or other carrier gases by means of optical devices. A beam oflight is directed toward a stream of the gas, and the amount of lightabsorbed from the incident beam is determined by measuring the intensityof the transmitted light, or the amount of light scattered from theincident beam in a transverse direction is measured. An abundance ofapparatus for performing either measurement is commercially available.

It is a common shortcoming of the known instruments that the readingsobtained on carrier gases containing equal weights of suspended solidsin a unit of volume differ significantly in response to differences inthe color, shape, index of refraction, and particle size of thesuspended material. As compared to particle size, the other parametersare relatively unimportant. At most, they can jointly affect the resultof a measurement by a factor of 2 or 3. Moreover, they can be heldreasonably constant for many installations'requiring control ofparticulates in emitted gases.

For an equal amount of particles suspended in a carrier gas, the valuesindicated by instruments measuring light absorption or light scatteringhave a distinct maximum at a particle size'of about 0.3 pt. Forparticles greater than 1 u, the instrument readings for visible lightare proportional to the actual weight divided by the particle diameter.If the particle size of the suspended solids varies by a factor of 20,an equal variation is indicated by the optical instruments even thoughthe weight of the suspended material and all other vari- 'ables are heldconstant.

Particles having diameters greater than 1 p. predomi-,

nate in the combustion gases discharged from power charged particlesfrom the carrier gas into the other gas, and the other gas is thenseparated from the carrier gas and subjected to measurement of suchoptical properties as absorption or scattering of light. As willpresplants and garbage incinerators equipped with filters loaded tocapacity, and the quantitative measurement of such particles in thefiltered combustion gases thus is necessary for monitoring the filters.Relatively complex and costly devices have been proposed for overcomingthe errors due to changes in particle size distribution which areinherent in the afore-described optical methods, but they have not beenfound entirely satisfactory.

The primary objects of the invention are a simple method ofopticallymeasuring the weight of suspended solid particles regardless oftheir size, and the provision of apparatus suitable for performing themethod, and capable of producing data which can be converted readily toreliable weight values even if the suspended particles are of unknownsize.

According to themethod of the invention, the soli particles suspended ina carrier gas are electrically charged. The carrier gas having thecharged particles suspended therein and another gas substantially freefrom suspended solid particles are then made to flow in contiguouslyadjacent parallel streams which are passed jointly through an electricfield transverse to the common direction of flow. The field must be ofsufficient strength to cause migration of a portion of the ently beshown, the readings of such properties are independent of particle sizeand uniquely correlated to the weight of the suspended particles perunit volume of the carrier gas so that the readings may be directlytranslated into weight per volume data by means of a calibration chart.

The apparatus necessary for performing the method may consistessentially of two elongated conduits having respective parallel,transversely juxtaposed portions separated by a common longitudinalwall. The wall has an opening, and a device for electrostaticallycharging the suspended particles in a carrier gas is arranged in one ofthe conduits upstream from the wall opening. Electrodes are offset fromthe opening transversely of the direction of conduit elongation, and theother conduit leads from the opening in the common wall toward themeasuring apparatus in a longitudinal direction away from theafore-mentioned charging device. In operating the apparatus, the carriergas to be tested is fed to the one conduit while another, relativelypure gas flows in the other conduit. The charged particles migrate inthe electric field through the wall opening and are then carried by theother gas to the measuring station.

Other features, additional objects, and many of the attendant advantagesof this invention will readily be appreciated as the same becomes betterunderstood by reference to the following description of a preferredembodiment when considered in connection with the appended drawing inwhich:

FIG. 1 diagrammatically illustrates the relationship of the size of anequal weight of suspended particles to the readings of scattered lightwhen carrier gas having fractions of uniform particle size suspendedtherein is subjected to analysis for light scattering;

FIG. 2 shows apparatus of the invention in fragmentary, sectionalelevation; and

FIG. 3 is a block diagram of the apparatus of FIG. 2.

In FIG. 1, the amount of scattered light I in linear units is plotted asa function of the logarithm of particle size, as determined with aconventional instrument for measuring light scattering by solidparticles in a carrier gas. The measurements on which the illustratedcurve are basedwere performed under otherwise uniform conditions,including the same ratio of particle weight to gas volume, for each ofthe particle fractions of uniform size that were tested. The gas wasair, and the particles were fly ash. The measurements were performedwith visible light. The plotted results are typical also of other gasesand types of particles.

wherein X; is a proportionality factor constant for the apparatus andmethod employed. The curve illustrates the difiiculties encountered inan attempt to measure the weight of particles suspended in a carrier gaswhen employing conventional methods.

Apparatus of the invention is shown in FIG. 2 only to the extend neededfor an understanding of the invention. The ilustrated apparatusessentially consists of two elongated conduits 1, 2 of rectangular crosssection whose illustrated portions are parallel and transverselyjuxtaposed. They are separated partly by a common wall 3. As indicatedby an arrow 4, a stream of the contaminated carrier gas is pumped orotherwise caused to flow through the conduit 1 under conditions in whichsuch flow is predominantly laminar. A stream of gas practically free ofsuspended particles, such as ordinary ambient air, is similarly causedto flow in the conduit 2 as indicated by the arrow 5. The velocities,pressures, and temperatures of the two gases should be as closelysimilar as possible.

A grid of wires 6 is arranged in th conduit 1 and connected to ahigh-voltage generator capable of charging the suspended particles inthe contaminated gas to saturation in a manner conventional inelectrostatic dust precipitators. The saturation charge of each particleis 0.75E,,D wherein E is the field strength in the charging zone and Dis the particle diameter.

Downstream from the grid 6, the common wall 3 has an opening 7 extendingover the full width of the wall, and the opposite walls of the twoconduits carry respective electrodes 8, 9 approximately coextensive withthe opening 7 and connected to a source of constant voltage, not shown,by leads 8', 9'. In the absence of an electric field between theelectrodes 8, 9, the two streams of gas flow into respective downstreamparts 10, ll of the conduits 1, 2 without mixing at the opening 7 to asignificant extent.

When the electrodes 8, 9 are energized, the charged particles moveacross the opening 7 from the contaminated gas stream in the conduit 1toward the pure gas stream in the conduit 2 as indicated by arrows 12.The velocity W of the charged particles transverse to the flow directionof the gases is constant, and according to Stokes law W E E v D whereinE, and D are as defined above, E is the strength of the field betweenthe electrodes 8, 9, and v is the viscosity of the two gases which maybe assumed to be equal and constant without introducing a significanterror. As long as E and E are held constant, the particles migratetransversely at a velocity which is directly proportional to theirdiameter.

The voltage applied to the electrodes 8, 9 must be chosen so that eventhe largest particles present are not precipitated, but remain suspendedin the air stream in the conduit part 11 when they leave the electricfield. A portion of the entire suspended phase is thus diverted from thecarrier gas in the conduit 1 into the pure gas in the conduit 2, themagnitude of the diversion being specific for each particle sizefraction. Particles of greater diameter are thus more numerous in theconduit part 11 relative to smaller particles than in the originalcontaminated gas. In other words, if the particles originally present inthe contaminated gas stream are considered as a plurality of fractionsconsisting each of particles of practically uniform size, each fractionis represented in the conduit part 11 in a concentration directlyproportional to the particle diameter in the fraction and to itsconcentration in the conduit 1. Assuming the particles in each fractionto be of uniform specific gravity, they are not only of uniformdiameter, but also of uniform weight.

The weight G of each fraction in a unit volume of gas in the conduitpart 11 may thus be represented by the equation G K2 D G wherein K is aconstant factor for the apparatus employed as long as all operatingvariables, such as voltages and flow velocities, are kept constant, andG is the weight of the fraction under consideration in a unit volume ofthe original contaminated gas entering the conduit 1.

The solid particles are mixed uniformly in the conduit part 11 with thestream of pure gas. The baffles employed for creating the desiredturbulence have been omitted since they are entirely conventional andneed to beplaced much farther from the opening 7 than can be shown inthe drawing, and the stream of initially pure air having solid particlessuspended therein is led by the conduit part 11 to the samenonillustrated, but conventional optical instrument by means of whichthe curve of FIG. 1 was established. As mentioned above, the amount ofscattered light J received by the instrument from the particles of eachfraction is a constant times the quotient of the particle weight dividedby the particle diameter:

and it has been shown that the weight of the particles of the fractionwhich reach the conduit part 11 is When the two last-mentioned equationsare combined, the light received by the instrument is found to be and afunction of G alone, that is, of the weight of the fraction originallypresent in the contaminated gas entering the conduit 1. The total lightreceived by the instrument is a function of the total 'weight of theparticles in the contaminated gas.

FIG. 3 shows a complete installation incorporating apparatus of theinvention'in sectional plan view, partly schematic. As in the apparatusshown in FIG. 2 the illustrated apparatus comprises two elongated inletconduit portions 1,2 which are parallel and transversely juxtaposed, andwhich are separated by a common wall 3. The common wall 3 has an opening7 and then separates two parallel and transversely juxtaposed outletconduit portions 10, 11. Again opposite walls of the conduits carryrespective electrodes 8, 9 coextensive with the opening 7 and sonnectedto a source 13 of constant voltage by leads 8, 9'.

Each conduit portion 1, 2, 10, 1 1 communicates with a respectiveseparate conduit 14, 15, 16, 17. Conduit 14 supplies ambient air free ofsuspended particles by means of a first air pump 18 followed by a dustfilter 19. Conduit 15 supplies a laminar stream of contaminated carriergas drawn from a dust chamber 20. Through conduit 16 a second pump 21exhausts a mixture of pure air and contaminated carrier gas through anoptical measuring instrument 22 whilst the remaining portion ofcontaminated gas is exhausted through conduit 17 by means of a thirdpump 23.

In a typical embodiment all pumps 18, 21, 23 have an air deliverycapacity of 30 litres per minute even if the pressure in the dustchamber, which may be a chimney or the like, slightly departs fromatmospheric pressure.

To charge dust particles in the contaminated gas drawn from dust chamber20, a charging device is disposed along conduit between dust chamber andconduit portion i. This device comprises an ion or electron dischargechamber 24 through which conduit 15 passes. In chamber 24 conduit 15 isinterrupted at two locations at which two annular discharge devices 25,for example corona discharge devices, are disposed which are connectedto a source 26 of high voltage by leads Instead of the corona dischargedevice shown device 25 may be an annular ion discharge device, negativeions being supplied in that case from. ion source 26.

To bind the ions injected into the contaminated gas stream to the dustparticles, the gasstream is conduit 15 passes through a charge portionof conduit 15 having two oppositely disposed electrodes 27 connected toa source 28 of alternating voltage by leads 27. In a typical case thealternating voltage of source 28 had a frequency of 1 kHz and was ofsuch magnitude that between electrodes 27 an alternating electric fieldwith a peak intensity of l kV/cm was prevailing. This and similardevices for charging particles in a contaminated gas stream are wellknown from electrical precipitators.

In a typical embodiment source 13 of constant voltage energizing theelectrodes 8, 9 was of such value that the strength of the electricfield between said electrodes was 500 V/cm. As described before inconnection with FIG. 2 the charged particles in the contaminated gasstream supplied by conduit 15 and conduit portion 1 move across theopening 7 toward the pure gas stream in the conduit 14 and conduitportion 2 to mix with the pure gas stream in conduit portion 11 andconduit 16. The optical dust measuring instrument 22 was a photometer ofthe type Sigrist UP 52B3/5TNF3 which has been calibrated with apolystyrene aerosol of l p. size. The flow velocity of the gas in theinstrument 22 does not influence the optical measurement.

What is claimed is:

1. In a method of measuring the weight of solid particles suspended in acarrier gas by optical means, the improvement which comprises:

a. electrically charging the solid particles suspended in said carriergas;

b. causing the carrier gas having the charged particles suspendedtherein and another gas substantially free from suspended solidparticles to flow in congration of a portion of said charged particlesfrom said carrier gas into said other gas;

d. thereafter separating said streams; and

e. determining an optical property of the separated other gas indicativeof the quotient of weight divided by the diameter of the particles insaid other gas as a measure of the weight of the solid particlesinitially suspended in said carrier gas.

2. In a method as set forth in claim 1, said optical property beingdetermined by measuring the light of a transverse incident light beamscattered by the particles suspended in said other gas.

3. A method as set forth in claim 1, wherein said particles in saidother gas have a diameter of at least one micron,

4. In an apparatus for determining the weight of solid particlessuspended in a carrier gas by means of optical measuring apparatus, theimprovement comprising:

a. first guide means for causing two streams of gas to flow in a commondirection in contiguously adjacent streams;

b. electrostatic charging means for electrically charging solidparticles suspended in one of said streams;

c. two electrodes offset from said two streams in opposite directionstransversely of the common direction of flow of said streams anddownstream from said charging means;

(1. means for applying an electric potential to said electrodes and forthereby generating an electric field between said electrodes in said twostreams; and I e. second guide means for separating said streams and forguiding the separated other one of said streams from said electric fieldto said measuring apparatus.

5. In an apparatus as set forth in claim 4, said first and second guidemeans including two elongated conduits having respective parallel,transversely juxtaposed por tions separated by a common longitudinalwall formed with an opening therein, respective parts of said portionsextending from said opening in opposite longitudinal directions, saidelectrostatic charging means being located in one of said conduits andoffset from said opening in one longitudinal direction, the part of theother conduit leading from said opening toward said measuring apparatusin a direction opposite to said one longitudinal direction, and saidelectrodes being oppositely offset from said opening for generating saidfield in said opening.

6. In an apparatus as set forth in claim 4, said first and second guidemeans being offset from said electric field in opposite directions.

1. In a method of measuring the weight of solid particles suspended in acarrier gas by optical means, the improvement which comprises: a.electrically charging the solid particles suspended in said carrier gas;b. causing the carrier gas having the charged particles suspendedtherein and another gas substantially free from suspended solidparticles to flow in contiguously adjacent streams in a commondirection; c. jointly passing said streams through an electric fieldtransverse to the common direction of flow of said streams, said fieldbeing sufficient to cause migration of a portion of said chargedparticles from said carrier gas into said other gas; d. thereafterseparating said streams; and e. determining an optical property of theseparated other gas indicative of the quotient of weight divided by thediameter of the particles in said other gas as a measure of the weightof the solid particles initially suspended in said carrier gas.
 2. In amethod as set forth in claim 1, said optical property being determinedby measuring the light of a transverse incident light beam scattered bythe particles suspended in said other gas.
 3. A method as set forth inclaim 1, wherein said particles in said other gas have a diameter of atleast one micron.
 4. In an apparatus for determining the weight of solidparticles suspended in a carrier gas by means of optical measuringapparatus, the improvement comprising: a. first guide means for causingtwo streams of gas to flow in a common direction in contiguouslyadjacent streams; b. electrostatic charging means for electricallycharging solid particles suspended in one of said streams; c. twoelectrodes offset from said two streams in opposite directionstransversely of the common direction of flow of said streams anddownstream from said charging means; d. means for applying an electricpotential to said electrodes and for thereby generating an electricfield between said electrodes in said two streams; and e. second guidemeans for separating said streams and for guiding the separated otherone of said streams from said electric field to said measuringapparatus.
 5. In an apparatus as set forth in claim 4, said first andsecond guide means including two elongated conduits having respectiveparallel, transversely juxtaposed portions separated by a commonlongitudinal wall formed with an opening therein, respective parts ofsaid portions extending from said opening in opposite longitudinaldirections, said electrostatic charging means being located in one ofsaid conduits and offset from said opening in one longitudinaldirection, the part of the other conduit leading from said openingtoward said measuring apparatus in a direction opposite to said onelongitudinal direction, and said electrodes being oppositely offset fromsaid opening for generating said field in said opening.
 6. In anapparatus as set forth in claim 4, said first and second guide meansbeing offset from said electric field in opposite directions.